Cushions generally are filled with down, feathers or foam. Down filling requires constant fluffing to avoid sagging. Pick up the cushion and shake it to redistribute the down. Feathers also require shaking and fluffing. Eventually feathers, unlike down, break and become crushed so they no longer fluff. They need to be replaced at this point. Foam eventually shows its wear by becoming compressed. Foam cushions sag where they receive the most use. Open the cushion and remove the foam pad. Replace it with new foam, and the cushion will look new again.
There is an easy and inexpensive solution to a sagging couch. Remove your seat cushions and measure the seating area. If you are not handy with a saw, take your measurements to a big box hardware retailer to cut a piece of plywood to your specifications. Place the board on your couch and replace the cushions on top of it. The plywood provides additional support while it prevents sagging. There is also a commercial set of vinyl planks available on the retail market. The planks are placed under the couch cushions and work in the same manner as the plywood support.
The springs in your couch eventually will show their wear by sagging. Their resilience lessens, and they no longer spring back into their original shape. Remove the fabric panel that covers the bottom side of your couch to expose the springs. You might be able to unscrew the individual springs, turn them upside down and screw them back into place. If the piece of furniture is expensive, have a professional upholsterer tackle this problem for the best results.
A cracked or rotted wood frame can be the source of the problem if your couch is sagging. Cracks can occur when large pets or kids jump on the couch, causing stress to the frame. Rotten wood occurs with older furniture that has been stored in damp places. Turn your couch over and remove the fabric panel on the bottom side to view the inside structure. If you discover a simple crack in the wood frame, screw a metal brace across the crack to provide support. If wood is rotted, the damaged piece will have to be removed and replaced. This could be a job for a professional upholsterer.